At the end of a snow storm, Emily saw there was a lot of snow on her front lawn. The temperature increased and the snow began to melt at a steady rate. There was a depth
of 15 inches of snow on the lawn when the storm ended and after 4 hours, the depth
of snow was down to 12 inches. Write an equation for S, in terms of t, representing
the depth of snow on Emily's lawn, in inches, t hours after the snow stopped falling.